The size of the fuse depends on the size of the wire the fuse protects (and perhaps the length of the wire). PERIOD. The fuse protects the wires from overload.
It does not matter if the auxiliary lights draw 20 amps. If the wire feeding the lights is a 16 gauge wire, installing a 20 amp fuse risks melting the 16 gauge wire. On the other hand, installing the proper fuse will result in the fuse blowing when the lights are activated.
All of my feeds to the distribution blocks are 10ga wire (to minimize I2R losses), and they're capable of handling much more current than the total safe accessory load. Thus, the original statement stands. Each individual accessory supply circuit is similarly spec'd and fused.
If you're running thin(ner) gauge wire to power high-amperage devices like audio amplifiers or driving lights, you're doing it wrong. You design for the load at the other end of the circuit then build in enough cushion that normal operation doesn't result in conductor heating. As an example: My spotlights are on their own relay-controlled circuit that uses a DPST automotive part and the run to the lights from the battery is done with fine-strand 10 or 12ga. No resistance losses here. I'm not concerned about saving one or two ounces of wire weight by using a smaller conductor. The fuse which protects this circuit is located on the battery side (before the relay) as close as possible to battery positive. It's rated 25% higher than the maximum current draw of the lights - which is significantly lower than the current carrying capacity of said wire.
Bear in mind that you have a little bit of leeway when designing accessory wiring as compared to the OEM, whose products are usually designed and manufactured at a price point; copper costs money. None of the OEM motorcycle wiring I've seen could ever be described as "overbuilt". It's "adequate" for the vehicle as-delivered.
ETA:
Here's a pictorial of the Fuze Block and related circuit:
The fuse in the supply side of the circuit shouldn't be protecting the "supply wire" itself. That wire should be capable of handling a decent margin above full rated accessory load without overheating. The fuse should be rated at the maximum incurred load plus a small percentage, assuming overall load is within alternator reserve capacity. (My main accessory supply fuses are 30A.) Individual circuits use a similar method. The idea is to place the protection
as close to the source as possible then let the Fuze Block's fuses handle the individual loads connected to them.
